This week brings us the
return of the famous Leonid Meteor Shower, a meteor display that over the past
several years has brought great anticipation and excitement to sky watchers
around the world.

Solely from the standpoint
of viewing circumstances, this will be a favorable year to look for these meteors,
since the Moon will be only a thin crescent and will have set in the west long
before the constellation Leo (from where the meteors get their name) begins
to rise. This is in stark contrast to 2002 when a nearly full Moon lit up the
sky like a spotlight and wiped out all but the brightest meteors.

Last year, conditions were
much better; the Moon had slimmed to a narrow crescent in the predawn morning
sky and offered little hindrance. But this year, the Moon will be out of the
sky completely, offering perfect viewing conditions.

The Leonid meteors are debris
shed into space by the Tempel-Tuttle comet, which swings through the inner solar
system at intervals of 33 years. With each visit the comet leaves behind a trail
of dust in its wake.

Lots of the comet's old
dusty trails litter the mid-November part of Earth's orbit and the Earth glides
through this debris zone every year. Occasionally we'll pass directly through
an unusually concentrated dust trail, or filament, which can spark a meteor
storm resulting in thousands of meteors per hour. That's what happened in 1999,
2001 and 2002.

Since the Tempel-Tuttle
comet passed the Sun in 1998, it was in those years immediately following its
passage that the Leonids put on their best show.

But now, the comet - and
its dense trails of dust -- have all receded far beyond Earth's orbit and back
into the outer regions of the solar system. So odds are that there is little,
if any chance of any unusual meteor activity.

Times of (prospective) peak
activity

In the 2004 Observer's Handbook
of the Royal Astronomical Society of Canada, meteor experts Margaret Campbell-Brown
and Peter Brown, indicate that this year's peak activity should occur on the
morning of Nov. 17.

They cite 9:00 GMT, which
corresponds to 4 a.m. EST and 1 a.m. PST. This is the moment when the Earth
will be passing closest to the orbit of the long-departed comet, and when our
planet seemingly is most likely to encounter some residual comet material. This
time is highly favorable for North Americans, especially those in the eastern
United States and eastern Canada.

But while Leonid rates are
unpredictable, it is unlikely that more than a dozen meteors per hour will be
seen this year during peak activity, at least for viewer's with dark skies away
from cities.

Other
meteor researchers, however, such as Jeremie Vaubaillon of France, David Asher
of Ireland and Esko Lyytinen of Finland have examined Leonid prospects for this
year and also suggest watching for some meteor activity two days later, on Nov.
19.

For instance: sometime around
6:40 GMT (1:40 a.m. EST) on the 19th, the Earth might interact with material
that was shed by comet Tempel-Tuttle back in the year 1333. But at best, only
about 10 Leonids per hour may be seen. Lyytinen even suggests that because of
gravitational perturbations affecting these particles after making 20 trips
around the Sun, that this sparse activity might actually take place several
hours earlier, which would favor observers in Europe.

Later that same day, the
Earth could possibly sweep through a fresher and somewhat more concentrated
trail of material dating back to 1733. Perhaps a brief bevy of 30 to 60 Leonids
per hour may seen, but the time frame when these meteors are predicted to be
most numerous (approximately 19:00 to 22:00 GMT) favors observers in Asia and
Australia (where it will be the early morning hours of Nov. 20).

How to watch

The meteors will appear
to emanate from out of the so-called "Sickle" of Leo, but prospective
viewers should not concentrate on that area of the sky around Leo, but rather
keep their eyes moving around to different parts of the sky.

Because Leo does not start
coming fully into view until the after midnight hours, that would be the best
time to concentrate on looking for the Leonid meteors.

The hours after midnight
are generally best for watching for "shooting stars" anyway, because
before midnight we are riding on the back side of the Earth in its orbit around
the Sun, whereas after midnight we are on the front or advancing side. After
midnight the only meteoroids escaping collision are those ahead of the Earth
and moving in the same direction with velocities exceeding 18.5 miles per second.
All others we will either overtake or meet head-on. But before midnight, when
we are on the backside, the only meteoroids we encounter are those with velocities
high enough to overtake the Earth.

Therefore, on the average,
morning meteors appear brighter and faster than those we see in the evening.

And because the Leonids
are moving along in their orbit around the Sun in a direction opposite to that
of Earth, they slam into our atmosphere nearly head-on, resulting in the fastest
meteor velocities possible: 45 miles per second (72 kilometers per second).
Such speeds tend to produce bright meteors, which leave long-lasting streaks
or trains in their wake.

After Friday morning, the
Leonids will tail off quickly. A few stragglers may be visible on subsequent
mornings.

Joe Rao
serves as an instructor and guest lecturer at New York's Hayden Planetarium.
He writes about astronomy for The New York Times and other publications, and
he is also an on-camera meteorologist for News
12 Westchester, New York.